1. Field of the Invention
The present invention relates to a rotary articulated robot and a method of controlling a rotary articulated robot and in particular relates to a rotary articulated robot having offset rotary joints and a method of control thereof.
2. Description of the Related Art
Rotary articulated robots have previously been proposed (for example PCT International Publication WO 88/03856) provided with offset rotary joints wherein the joints have axes of rotation inclined at a prescribed angle with respect to the link axis. These offset rotary joints have a construction in which the driven arm performs conical rotary movement with an offset(slant) angle, its vertex being the point of intersection of the arm axis and the offset rotary axis. By providing a plurality of these offset rotary joints, precise three-dimensional positional location of the end effector can be achieved over a wide range of movement, with a simple mechanism involving only rotary movement. Also there is the advantage that, since only axial rotation is involved, precise positional location control can be achieved easily and large torque can be transmitted. Also, by linking a large number of arms by offset rotary joints, the arm as a whole can be made to move in a snake-like manner; thus, the end effector can be moved with a complicated operational path, making it possible to achieve movement that was not possible with previous rotary articulated robots.
However, when a large number of arms are linked in order to implement the above function effectively, the weight of the arm itself becomes large, giving rise to problems such as that the payload of the end effector is reduced and it becomes difficult to perform high-load operations. An offset rotary joint is therefore sought whereby higher torque can be obtained with lighter weight and smaller size.
In conventional robot systems, known methods of teaching position and attitude information include the direct teaching method, the remote teaching method using a joystick etc, and off-line teaching using numerical value data input etc. These methods of teaching are methods in which points on the trajectory of the robot from its starting point to the target position are directly or indirectly successively taught. In order to move the robot along the designated path between the operating points that have been taught, control quantities for the motors arranged at each joint are calculated in order to generate a trajectory, and these control quantities are supplied as instruction values to the motors. Consequently, when a complicated track is generated by a rotary articulated robot, there are the problems that teaching is difficult and calculation for generating the track becomes complicated and extensive, resulting in increased load on the computer, and slowing down the response speed and as a result of making it impossible to achieve smooth movement.
In particular, in the case of a rotary articulated robot having offset rotary joints as described above, there are countless combinations of movement of the joints in order to move the end effector from its starting point to the target position and generation of a trajectory is complicated so with the conventional method of controlling a robot the response time was inevitably slow.
An object of the present invention is to provide a rotary articulated robot of high functionality in which stronger rotary torque can be transmitted by adopting drive motors of smaller size and in which high precision positional location can be achieved, wherein offset rotary joints of small size, light weight and higher torque are obtained, and, by a linking up these offset rotary joints in multiple stages, large payloads can be achieved and a wide range of movement, and with which complicated precise movements are possible.
Another object of the present invention is to provide a method of controlling a rotary articulated robot in which, even with a rotary articulated robot having an end effector that performs complicated movement as described above, a trajectory to the target position can be generated in a simple fashion without requiring complicated calculation processing on each occasion, making it possible to perform a task with a rapid speed of response and in a smooth fashion even though the movement involved is complicated.
In order to achieve the above objects, in a rotary articulated robot according to the present invention having at least a plurality of offset rotary joints wherein a drive arm and driven arm are driven in rotation about an offset rotary axis inclined with respect to the arm axis, a hollow rotary shaft that is driven in rotation by a motor is arranged rotatably, being inclined at a prescribed offset angle, at the leading end of either the drive arm or the driven arm, while a rotor member to which turning effort from the hollow shaft is transmitted is fixed at the base of the other arm, said hollow rotary shaft and said rotor member constituting a high reduction ratio transmission/torque increasing mechanism.
Suitably, a harmonic drive mechanism can be applied as the high reduction ratio transmission mechanism, but there is no restriction to this, and other planetary gearwheel reduction mechanisms or special tooth shape reduction mechanisms etc could be employed. Also, preferably, by arranging for the turning effort from the motor to the hollow rotary shaft to be transmitted by meshing of an externally toothed bevel gear fixed to the output shaft of the motor and an internally toothed bevel gear fixed to the base of the hollow rotary shaft, it is possible to arrange the rotary axis of the motor within the arm coincident with or parallel to the arm axis and the arm diameter can thus be made smaller. Also, as the transmission mechanism from the motor to the hollow rotary shaft, a belt transmission may be adopted, comprising a pulley fixed to the motor output shaft, a pulley fixed to the base of the hollow rotary shaft, and a timing belt arranged between this pulley and the aforesaid pulley.
By making the motor of integral-case form such that its case constitutes part of the arm and by using a motor fitted with a brake and in which the encoder and tachometer are integral therewith, mounting and assembly on to the main body of the arm are facilitated and maintenance is also facilitated. Also, the arm can be held in a prescribed position without requiring a special brake device. Furthermore, by making the shaft end on the opposite side to the output side of the motor shaft project and providing a manual rotary adjustment portion on the projection, angular adjustment during assembly can be performed manually, facilitating assembly.
In a method of controlling a rotary articulated robot according to the present invention whereby the second object is achieved, the operating region of the end effector is divided into a plurality of blocks, the operating conditions of each joint necessary for movement to a prescribed block are converted to database form as block region data for each block, the teaching of operating points in the prescribed block region is converted to database form as in-block operating point data, and the end effector is moved to a reference point of the selected block by driving the joints so as to satisfy these conditions. The operation of the joint is then determined by fetching the in-block operating point data when the reference point of the selected block is reached. Consequently, since the movement of the joints is linked to the movement of the end effector and no complicated calculations to determine the track need be performed, the amount of data processing can be considerably reduced, making it possible to speed this up.
Although, in the present invention, as described above, the block region data and also operating point data within the block regions are converted into database form and control is performed using these in combination, thereby making it possible to speed this up, it would be possible to convert only either one of these to database form. In order to move the end effector into a prescribed block region, at each joint, the block region data provided from the database and the current operating condition are compared, and the joints are independently operated in parallel by independently controlling the motors such that the block region data is satisfied. If the operating region extends over a broad region, a plurality of blocks may be combined.
Also, when the arm of the rotary articulated robot is made to perform a complicated winding movement, in order to lighten the load on the computer so that higher speed can be achieved, according to the present invention, the joint control means that are installed at each joint are respectively independently connected with the central controller that controls the movement of the robot as a whole, and a network such that direct exchange of data can be achieved is constituted between the joint control devices; thus the joints can be controlled in parallel and angle information of the joints can be directly sent to adjacent joints when complicated movements are performed.